The lack of organic matter content in deteriorated soils in drought stricken areas is a serious problem for vegetation cover. The use of organic amendments like biochar and compost to agricultural soils could be a good remedy for poor soil, water deficiency and plants growth. The comparative analysis of the effects of biochar produced from argan shells, compost and their mixture on the growth of quinoa (Chenopodium quinoa), sugar beet (Beta vulgaris) and alfalfa (Medicago sativa) was the main objective of this research. It was assumed that the addition of biochar mixed with compost could enhance soil fertility and then improve crop productivity of plants. For this purpose, three treatments (2%, 4% and 6%) of biochar (B), compost (C) and biochar–compost (B–C) mixture were applied to soil. The samples for different treatments were collected, incubated and then analyzed. In order to evaluate the plant growth, the greenhouse experiments were conducted for three months in the pots filled with untreated and treated soils. The results showed that the addition of biochar and compost to soil increased pH, electrical conductivity, exchangeable cations, total nitrogen, phosphorus, total organic matter and cation exchange capacity. A significant improvement of the biomass of plants has been observed when applying a mixture of biochar and compost at 6% for quinoa and sugar beet and at 4% for alfalfa. According to the obtained results, mixing argan biochar with compost has a positive impact on soil nutrients and growth of plants.